In a fuel injection device of a Diesel engine, an actuator is connected to a fuel passage which connects a fuel injection pump with an injection nozzle, or to a fuel injection pumping chamber, to control the fuel pressure. A piezoelectric laminate used in this actuator is advantageous in that its response characteristic is good and that it affords a large driving force.
In general, a piezoelectric laminate is used in a preset load-applied state in order to attain the improvement of performance and reliability. It is known, for example, in Japanese Patent Laid-Open Nos. 10178/84 and 150067/87 that a coned disc spring, a coil spring, or a bolt, is used as a unitary member for the application of the said preset load.
In the structures of known piezoelectric laminates, however, many hours of use may result in a shortening of the overall length in the laminated direction due to compatibility of the constituent piezoelectric elements or due to domain inversion. If in view of this point there is used a piezoelectric type actuator wherein a preset load is applied to a piezoelectric laminate by means of a coned disc spring having a large spring constant or by virtue of a tightening force of a bolt, since the effective tightening stroke of the bolt which fulfills the spring action is small, there arise problems, for example, the preset load is apt to decrease and the performance of the piezoelectric elements is deteriorated, or in the extreme case the preset load is extinguished, causing damage to the piezoelectric elements.
Conversely, according to a piezoelectric type actuator wherein a preset load is applied using a spring having a small spring constant and a large effective stroke, e.g. a coil spring, it becomes necessary to thicken the coil spring in diameter for the application of a predetermined preset load (0.5-5 kg/mm.sup.2), so there arises the problem that the size of the piezoelectric actuator becomes larger.
The present invention has been accomplished in order to solve such problems, and it is the object thereof to provide a piezoelectric type actuator wherein a predetermined preset load is applied in an initial state and which permits the application of a minimum preset load required under any environmental conditions during use and can be reduced in size.